The present invention relates to a hydrualic booster in general, and particular to a hydraulic booster for the actuation of a master cylinder arrangement of a vehicle brake system.
Various hydrualic brake booster constructions are already known, among them such including a booster piston and a stationary brake valve which is located separately from the booster piston and which serves for the selective control of the communication between a pressure chamber and either a pressure source or an unpressurized reservoir. In such constructions, there is usually interposed between a pedal-operable operating element or piston rod and a control device of the brake valve a device operative for the transmission of the control stroke of the piston rod. Resilient means may further be provided for causing the brake valve to adopt a position in the brake-released position in which the pressure chamber and the unpressurized reservoir are in communication.
Hydraulic brake boosters are known and widely used for the actuation of master cylinder pistons of vehicular brake systems, in which the brake valve is arranged in the booster piston. The booster piston has a circumferential groove which, together with the housing, forms an annular chamber communicating with the pressure source. Upon the application of force to the piston rod, which is coupled with the valve spool of the brake valve, the valve spool will be displaced relative to the brake valve housing so as to interrupt the communication between a pressure chamber and an unpressurized reservoir and to bring radial bores in the valve housing and in the brake valve into coincidence, so that pressurized medium enters the pressure chamber. An important disadvantage of such hydraulic brake boosters is the need of providing sealing means at the booster piston which are permanently exposed to the pressure of the pressure source. Such high-pressure seals exert considerable frictional forces on those parts which are in contact with them; thus a relatively high pressure must be initially built up in the pressure chamber before any movement of the booster piston occurs. During this stage of control, a considerable reaction force caused by the pressurization of the piston rod may already be felt at the brake pedal, although no braking pressure has been built up as yet in the master cylinder arrangement. Once the pressure required for the breaking away of the booster piston has been built up in the pressure chamber, the booster piston will abruptly start moving and effect a correspondingly quick build-up of a relatively high braking pressure in the master cylinder arrangement and hence in the brake system. This will interfere with smooth and convenient operation in the range of relatively weak braking actions as a sensitive control of the braking force is hardly possible in this range. Relatively strong compression springs are required for the return of the booster piston since they must be able to overcome the adhesive forces of the high-pressure seals. Such springs are relatively heavy.
A brake booster of this type which, however, avoids the aforementioned disadvantages is disclosed in the German published patent application DE-OS No. 23 64 111. This hydrualic brake booster has a piston rod connected with a brake pedal, the end of the piston rod which faces away from the brake pedal projecting into the booster chamber and being received by a blind bore of the booster piston. The booster piston has an extension. A lever is supported on this extension in a tiltable manner; the other end of this lever is fastened to the brake valve. A pivot is provided between the point of support of the lever and the lever end which is fastened to the brake valve. The pivot is supported on a holding device which is displaceable on the piston rod and which is kept in a pedal-remote position by force of a spring.
In this known arrangement it must be considered a disadvantage that the axes of the brake valve and of the booster piston can be only be arranged substantially parallel to each other or, in other words, that it is impossible to freely select the angle enclosed by these axes.